1. FIELD OF THE INVENTION
The present invention relates to an improved diaphragm type pump device to effect a pumping function by reciprocating movement of a diaphragm.
2. DESCRIPTION OF PRIOR ART
An air pump as an example of a diaphragm type pump device which has been generally known will be described with reference to FIG. 1.
In FIG. 1, an upper casing 2 provided with an intake port 4 and a discharge port 5 in the top surface is assembled with a lower casing 3 provided with an opening at the bottom surface to provide an inner space. A diaphragm 1 is held in the inner space with the outer circumferential part clamped by flanges of the upper and lower casings 2, 3, whereby an operation chamber (A) is formed in association with the upper casing 2 and the diaphragm 1. The operation chamber (A) is communicated with the outside through the intake port 4 in which a check valve 6 is arranged and the discharge port 5 in which a check valve 7 is arranged. The direction of communication of the check valve 6 is opposite to that of the valve 7. The diaphragm 1 is connected to a connecting rod 8 through reinforcing plates 9. The connecting rod 8 is driven by a driving source (not shown) in the direction indicated by arrow marks to effect reciprocating movement of the diaphragm 1.
In the diaphragm type pump device having the above-mentioned construction, when the connecting rod 8 is moved in the direction of a as shown by an arrow mark from the state as in FIG. 1, the diaphragm 1 is also moved in the direction of a. Accordingly, the volume of the operation chamber (A) is increased with the result of reduction in the pressure in the operation chamber, whereby the check valve 7 is closed, while the check valve 6 is opened to suck air through the intake port 4.
When the connecting rod 8 is moved in the direction of b to return to the state as shown in the FIG. 1, the diaphragm 1 is also moved in the direction of b. Accordingly, the volume of the operation chamber (A) is decreased and pressure in the operation chamber is increased, whereby the check valve 6 is closed, while the check valve 7 is opened to discharge air through the discharge port 5.
In the conventional pump device, however, the volume of the operation chamber (A) does not become zero even when the diaphragm 1 is at the upper dead point as shown in FIG. 1 in the reciprocating movement and there is a large amount of residual air in the operation chamber. Since the residual air is subjected to compression and expansion, change in pressure in the operation chamber is small in comparison with the distance of movement of the diaphragm 1. Therefore, efficiency of the pump device is low. Further, when the pump device is used for a compression pump or a vacuum pump, an achievable degree of vacuum is low.
To eliminate the disadvantage, there is an attempt of reducing a residual volume by changing the shape of the upper casing 2. However, when the residual volume is made too small, there is risk of damaging movable parts such as the diaphragm 1, the connecting rod 8 by impingement against the upper casing 2. Accordingly, reduction in the residual volume undegoes dimensional restriction.